It is well known that the need for nitrogen for inert atmospheres in the metallurgical and other industries has been a principal factor in the development of tonnage cryogenic air separation plants. Accordingly, such air separation plants have been designed to produce high purity nitrogen, but a large portion of the separated nitrogen has been required to be used as a waste stream to remove the water and CO.sub.2 both of which are frozen out of the feed air stream in the main reversing heat exchangers. Thus, only about 50% or less of the nitrogen contained in the feed air could be recovered as product nitrogen.
More recently, there has been an increasing demand for gaseous nitrogen in the chemical process industries wherein nitrogen is used in blanketing operations and other applications. Where the purity level requirements do not justify the cost of the ultrahigh purity nitrogen normally produced by the prior art cryogenic plants, non-cryogenic techniques for producing nitrogen have been used. Thus, nitrogen has been recovered from air by consuming the oxygen therein in a combustion chamber using natural gas, oil, or the like, as a fuel followed by further treatment to remove most of the carbon dioxide and water so as to produce a product nitrogen stream containing tolerable amounts of water and carbon dioxide. Although such combustion processes generally require a smaller capital investment than conventional cryogenic air separation plants, the operating costs of combustion processes have increased significantly because of the recently increased cost of the fuels required for the combustion step. At the same time, the need for nitrogen of higher purity than that offered by such combustion processes has also increased. As a result of these factors, a serious need has arisen for tonnage air separation plants which are capable of recovering larger volumes of high purity nitrogen at lower cost than conventional cryogenic plants.